Identifying Ions

• How would you carry out a flame test?

  Clean a wire loop in HCl to remove unwanted ions, dip the loop into your sample, place wire with sample in flame, record the colour of the flame.
• How would you distinguish between aluminium ions and calcium ions?

  Add excess NaOH, Al(OH)3 precipitate reacts to form a colourless solution, Ca(OH)2 precipitate remains unchanged.
• Describe the test for ammonium ions

  Add dilute NaOH to the sample and warm, ammonia gas is produced which turns damp red litmus paper blue.
• Describe the test for carbonate ions

  Add dilute acid, pass gaseous product through limewater, CO2 turns limewater cloudy.
• Describe the test for sulfate ions

  Add a few drops of dilute hydrochloric acid then a few drops of dilute barium chloride solution, white precipitate forms.
• Why is acid added before testing for sulfate ions, why can’t sulfuric acid be used?

  Carbonate ions react with barium chloride to produce a white precipitate, acid react with CO3(2-) ions to prevent a false positive result. Addition of sulfuric acid would introduce sulfate ions to the sample giving a false positive result.
• Describe the test for halide ions

  Add dilute nitric acid to react with carbonate ions so no Ag2CO3 forms (white solid). Add silver nitrate, precipitate forms:

  White: AgCl

  Cream: AgBr

  Yellow: Agl
• Describe the test for carbon dioxide

  Forms a white precipitate for calcium carbonate or turns limewater from colourless to cloudy.